1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of image bearing members disposed along an intermediate transfer member or a recording material conveying member, and more particularly to control of adjusting a rotation phase positions between the two image bearing members with different driving motors.
2. Description of the Related Art
A tandem-type multi-color image forming apparatus has been widely used, in which image forming portions provided with developing colors of cyan, magenta, yellow, and black are disposed along the intermediate transfer member or the recording material conveying member (rotating member).
In some of the tandem-type image forming apparatus, referring to FIG. 1, an image forming portion Pd for black and an intermediate transfer belt (104) are driven by a common driving motor (102d) while the other image forming portions (Pa, Pb, Pc) for the other colors are driven by the other driving motor (111). This is because the image forming portions (Pa, Pb, Pc) for yellow, magenta, and cyan can be easily stopped when performing a black monochrome mode in which monochrome images are formed using only the image forming portion Pd for black (U.S. Pat. No. 6,173,141).
However, in order to reduce a color deviation in respective colors in a full color image, it is preferable that photosensitive drums (101a, 101b, 101c, 101d) for respective colors equally reproduce the rotation phase each time of image formation. In this case, the three photosensitive drums (101a, 101b, 101c) have rotation phases aligned by the common driving system, so that synchronization is required in a rotation phase between these drums and the photosensitive drum (101d) for black which has an independent driving system.
U.S. Pat. No. 6,173,141 discloses such a tandem-type multi-color image forming apparatus that an image bearing member for black and a recording material conveying belt are driven by a driving system which is independent from a driving system for image bearing members for cyan, magenta, and yellow. Herein, speed fluctuations are detected in the image bearing members for cyan, magenta, and yellow and in the image bearing member for black, and the image bearing members for cyan, magenta, and yellow keep synchronizing their rotation phases so that phases of the speed fluctuations are aligned between the image bearing members for cyan, magenta, and yellow and the image bearing member for black.
In a case of using a motor by which a rotation phase is controlled with high accuracy at a time of gathering speed at start-up and reducing speed at shutdown, unlike the image forming apparatus disclosed in U.S. Pat. No. 6,173,141, it is not necessary to keep synchronizing rotation phases during operation. The rotation phases may be synchronized between two driving systems at an image formation interval between prior rotation before image formation, subsequent rotation, and detection of a significant deviation between rotation phases. Rotation is stabilized during image formation by synchronizing rotation phases at once at a time of image non-formation, thereby preventing occurrence of color derivation resulting from control of synchronizing the rotation phases.
However, in a case of performing control of synchronizing the rotation phases of the image bearing member for black and the image bearing members for the other colors at once, friction is caused between the image bearing members for the other colors and the rotating member with a velocity difference in association with the control of synchronizing the rotation phase. In a case where phase synchronization is always repeated at approximately the same phase positions in the image bearing members for the other colors, friction may proceed or many sliding-contact damages may be caused at the specific positions on the periphery of the image bearing members for cyan, magenta, and yellow. Therefore, variations in concentration or image damages appear prominently in a biased manner at one part of the output image, thereby deteriorating an image quality at an early stage in life of the image bearing members.